As disclosed in 2001-239861A and 2002-192996A, various types of power seat slide devices for a vehicle, which adjust a position of a vehicle seat relative to the vehicle floor by moving an upper rail so as to slide relative to the lower rail by means of power generated by an electric driving source, has been proposed. FIG. 5 illustrates a longitudinal sectional view indicating an example of the power seat slide device for a vehicle. As shown in FIG. 5, the power seat slide device includes a lower rail 81, an upper rail 82, a nut member 83, a screw shaft 84 and a gear box 85. Specifically, the lower rail 81 is fixed to the vehicle floor, and the upper rail 82 supports the vehicle seat in a manner where it is supported by the lower rail 81 so as to be slidable. The nut member 83 is fixed to the lower rail 81, the screw shaft 84 is screwed into the nut member 83, and the gear box 85 is fixed to the upper rail 82 at a front end thereof in order to transmit rotations generated by the electric driving source.
The gear box 85 includes a gear housing 86 made of resin, a supporting bracket 87 and a plate 88, which are made of metal plates in order to hold the gear housing 86. In a gear housing 86 of the gear box 85, a worm 91, which is provided at the electric driving source, and a helical gear (worm wheel) 92, which is made of resin and fixed to a top end portion of the screw shaft 84 so as to mesh with worm 91, are housed.
The helical gear 92 is fixed to the screw shaft 84 within the gear box 85 in a manner where it is sandwiched between a washer 93 and a nut 94. The washer 93 is made of metal so as to engage with the screw shaft 84, and the nut 94 is fastened to the bolt portion 84a of the screw shaft 84, which is provided so as to penetrate through the gear box 85.
The gear box 85 is fixed to the upper rail 82 in a manner where a supporting bracket 87 engages with a front end surface of the upper rail 82, and plural attachment portion 87a of the supporting bracket 87 are fixed to the upper rail 82 by means of rivet 95. The attachment portions 87a are formed so as to extend in the side of the upper rail 82, and only one attachment portion 87a is illustrated in FIG. 5.
In this configuration, when the screw shaft 84, to which the helical gear 92 is provided, is rotated by rotating the worm 91 by means of the electric driving source, the screw shaft 84 is moved in an axial direction thereof relative to the nut member 83 fixed to the lower rail 81, as a result, the upper rail 82 slides relative to the lower rail 81 so that the position of the vehicle seat relative to the vehicle floor F is adjusted.
A condition where the upper rail 82 is supported by the lower rail 81 will be explained in accordance with FIG. 6. FIG. 6 illustrates a cross section of the power seat slide device illustrated in FIG. 5 along VI-VI line. For the sake of convenience, the screw 84 (bolt portion 84a) and the nut 94 fastened to the screw 84 are also illustrated in FIG. 6. As shown in FIG. 6, the lower rail 81 includes a bottom portion 81a, two first side wall portions 81b each of which extends upward from each end of the bottom portion 81a, and two first folded wall portions 81 extend each of which extend inward from each upper end of the first side wall portions 81c, and further extend downward toward base end of the first side wall portion 81b. Further, a resin shoe 96 is attached at each end portion of the first folded wall portions 81c so as to extend within entire length in a longitudinal direction thereof.
On the other hand, the upper rail 82 includes a cover portion 82a, a pair of second side wall portions 82b and a pair of second folded wall portions 82c. Specifically, the second side wall portion 82b extends downward from each end portion in a width direction of the cover portion 82a, and the second folded wall portion 82c extends outward and further extends upward so as to be surrounded by the first side wall portion 81b and the first folded wall portion 81c. 
In a manner where the resin shoe 96 is attached to the second folded wall portion 82c of the upper rail 82, the upper rail 82 is supported on the rollers 97 provided between the second folded wall portion 82c and the bottom portion 81a of the lower rail 81. Thus, the upper rail 82 is supported so as to be slidable relative to the lower rail 81 in a manner where the rollers 97 are rotated between the upper rail 82 and the lower rail 81, and the second fold wall portion 82c is guided by the resin shoe 96.
In such configuration, in order to avoid interference with the lower rail 81, the supporting bracket 82 needs to be formed so as to avoid a space for providing, such as the resin shoe 96.
Thus, the supporting bracket 87 engages with a top end surface of the cover portion 82a of the upper rail 82 and with an upper part of a top end surface of the second side wall portion 82b, and the upper rail 82 is not extended downward so as to engage with a lower part of the second side wall portion 82b. 
Thus, when an impact load is applied to the upper rail 82 and the supporting bracket 87 (gear box 85), the supporting bracket 87 or the like deforms due to the load intensively applied to the engaging surface (load transmitting surface), which is unevenly distributed at the upper rail 82, as a result, the vehicle seat may be displaced.
Especially, because the supporting bracket 87 (gear box 85) engages with the upper rail 82 at the upper portion thereof relative to the screw shaft 84 that penetrates through the supporting bracket 87, the load is intensively applied the engaging surface (load transmitting surface), and the supporting bracket 87 is bent downward, as a result, the vehicle seat may be displaced.
A need thus exist to provide a power seat slide device for a vehicle that reduces chances of positional displacements of the vehicle seat cause by an impact load.
In this specification, the vertical direction may not mean an accurate vertical direction.